AZA is classical immunomodulator,often applied to hematologic diseases and immune-related diseases,also the most commonly used drug in the maintenance of remission in Crohn's disease, it is indicated in steroid resistant or dependent patients, in those whose frequency of relapse is >1 per year, in patients after induction of remission with IFX, and in patients whose remission were induced by surgical resection. However, AZA may cause some adverse effects,the most serious adverse effect is leucopenia, which can develop suddenly and unpredictably, though it is rare (around 3%).